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Optical data highways carry quantum information

24 Jun 2013

Vienna-based scientists say new technique stores quantum information for long enough to create optical global quantum networks.

Researchers at the Vienna University of Technology have demonstrated “quantum mechanical coupling” of atoms to glass fiber cables. Now, they say that their technique is enabling storage of quantum information for a sufficiently long period of time to achieve global quantum networks based on optical fibers.

The associated scientific article first appeared in Physical Review Letters.

The researchers led by Arno Rauschenbeutel (of Vienna Center for Quantum Science and Technology and Institute of Atomic and Subatomic Physics of the Vienna University of Technology) have now demonstrated experimentally that such glass fiberscan store quantum information long enough so that they could be used for entangling atoms hundreds of kilometers apart. This constitutes a fundamental building block for a global fiber-based quantum communication network.

Rauschenbeutel commented, “In our experiment, we connect two different quantum physical systems. On the one hand, we use fiber-guided light, which is perfect for sending quantum information from A to B, and, on the other hand, we rely on atoms, which are ideal for storing this information.”

By trapping atoms at a distance of about 200nm from a glass fiber, which itself only has a diameter of 500nm, a strong interaction between light and atoms can be effectted. This allows the exchange of quantum information between the two systems. This information exchange is the basis for technologies such as quantum cryptography and quantum teleportation.

Currently, there are different approaches towards performing quantum mechanical operations and exchanging quantum information between light and matter-based memories. However, for many of these systems it is challenging to store and to retrieve the information efficiently.

Standard fiber

The method developed at the Vienna University of Technology overcomes this problem: “Our setup is directly connected to a standard optical glass fiber that is nowadays routinely used for the transmission of data,” says Rauschenbeutel.

“It will therefore be easy to integrate our quantum glass fiber cable into existing fiber communication networks.” In the past, the researchers already demonstrated that atoms can be controlled and efficiently coupled to glass fibers. But the suitability of the fiber-coupled atoms for storing quantum information and for long-distance quantum communication had long remained a challenge.

After some time, the quantum information stored in the atoms is lost as it leaks into the environment due to an effect known as “decoherence”.

“Using some tricks, we were able to extend the coherence time of the atoms to several milliseconds, in spite of their small distance from the fiber surface,” explains Rauschenbeutel.

Light in glass fibers travels about 200km/ms. As the light carries the quantum information, this defines the separation that could be bridged with such a system via the entanglement of atoms.

Even in regular glass fiber-based telecommunication, the range of light propagation is limited: the longer the fiber, the weaker the signal. To overcome this problem, repeater stations are inserted into the network. They amplify the optical signals after a certain distance. In this way, global communication becomes possible.

This simple concept of signal amplification cannot be implemented in quantum mechanics. It is nevertheless still possible, albeit more involved, to build so-called “quantum repeaters”. They can be used to link several shorter sections to one long quantum connection.

Arno Rauschenbeutel is confident that his technique holds great promise: “By using our combined nanofiber-atom-system for setting up an optical quantum network including quantum repeaters, one might transmit quantum information and teleport quantum states around the world.”

About the Author

Matthew Peach is a contributing editor to optics.org.

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